INTRODUCTION

A knowledge of the growing characteristics of guayule (Parthenium argentatum
Gray, Asteraceae) shrubs, with details of rubber and coproducts production and
response to management practices is important for the selection of high yielding
plants (Macrae et al. 1986; Dierig et al. 1989a,b; Foster and Moore 1991; Foster
et al. 1991; Nakayama 1991; Nakayama et al. 1991; Jasso de Rodríguez
et al. 1996), as well as to define the best time for harvesting. This information
would be useful to maximize commercial rubber and coproducts production. Several
studies have been carried out looking for rubber content variation throughout
the year and the possibility to increase rubber yield. There are reports suggesting
a relationship between rubber synthesis and environmental factors or agronomic
practices (Appleton and Van Staden 1989; Benedict 1991; Estilai 1991; Foster
and Moore 1991; Nakayama 1991; Nakayama et al. 1991; Ray et al. 1992; Jasso-Cantú
et al. 1997). However, these reports did not present explicit relationship among
the variables.

Other authors have reported that rubber percentage in guayule decreases throughout
the growth cycle (Hammond and Polhamus 1965; Downes and Tonnett 1985; Appleton
and Van Staden 1989; Macrae et al. 1986; Nakayama 1991; Jasso-Cantú et
al. 1997). There has been speculation about the causes promoting this reduction
but no evidence has been presented. We proposed the existence of rubber synthesis
periods (Jasso-Cantú et al. 1997) with a multimodal molecular weight
distribution curve (Angulo-Sánchez et al. 1995), but there are no reports
confirming the relationship between the growth and yield variables and rubber
production cycles. This paper reports results from a three-year investigation
relating rubber and resin content and biomass production to temperature and
rainfall in two Mexican guayule accessions and one check cultivar developed
at Universidad Autónoma Agraria Antonio Narro in Saltillo, Coahuila,
México.

METHODOLOGY

Three high rubber yielding accessions were used for the study: BG1123 and
BG1132 from the state of Durango and BG11605 with rubber contents of 11.07%,
10.97%, and 8.8% respectively (Lopez and Kuruvadi 1985; Kuruvadi 1988; Angulo-Sánchez
et al. 1995).

Details on the experimental procedures for the seeding and cultivation for
these accessions were reported by Jasso-Cantú et al. (1997). The three
genotypes were seeded during May 1992 in a greenhouse, transplanted to an experimental
field in October in a randomized block design with three replications (the plant
density was 15,625 plants/ha) and grown under rainfed conditions, with no supplemental
irrigation. Three different plants from each accession were sampled every month
for dry weight, rubber, and resin content. Analyses for rubber and resin followed
procedure described by Jasso de Rodríguez et al. (1993). Rainfall and
temperature were recorded during the study period. The thermal units (TU) were
calculated according to Jaafar et al. (1993). They were accumulated during the
experiment period as cumulative TU (STU).

The relationship among dry weight, rubber, and resin content as a function
of time, were analyzed and their function defined according to the following
equations.

Dry weight was divided by a weight factor of 10 to handle quantities of similar
magnitude. The fractions are presented in a plot to evaluate their relative
dependence.

Cumulative thermal units (STU), rainfall (Rf),
and rubber yield (Rby) relative variables were defined to construct ternary
diagrams allowing for relationship between the three monitored variables. Normalized
variables and their fraction were calculated according to the following equations:

fSTU = (STU
/ 100) / (STU / 100 + Rf / 10 + Rby) [5]

fRf = (Rf / 10) / (STU
/ 100 + Rf / 10 + Rby) [6]

fRby = Rby / (STU
/ 100 + Rf / 10 + Rby) [7]

where

Rby = Dw * Rubber content (%) [8]

The fractions fulfill the condition stated in Eq. 4. In these equations STU
and Rf were divided by weight factors of 100 and 10, respectively, to obtain
good point spacing.

Relationship among the measured variables were estimated using the equation
proposed by Miller et al. (1958) with a statistics software MStat (MStat-C 1990).
The T-test (Manly 1990) was used to determine differences among the variables
mean values.

Accession BG1123 is used as an example to evaluate the relationship among
rubber content and dry weight as a function of climatic conditions as well as
for relative variables relationship versus time. The data for the three accessions
was used to construct ternary diagrams, relating temperature, rainfall, and
rubber content.

RESULTS AND DISCUSSION

Climatic Data

Temperatures recorded during the experiment period are presented in Fig. 1.
Less variation occurred in the maximum temperature values compared with the
mean and minimum values. The maximum temperature occurred in July and the minimum
between DecemberFebruary. The minimum temperatures were below 0°C
for all three years.

Fig. 1. Temperatures recorded during three years for the guayule study.

Rainfall was plotted as a function of the experiment elapsed time (Fig. 2).
During the first 13 months, rainfall was lower than 30 mm/month, which is common
for this arid region. From months 14 (June) to 17 (September) the highest rainfall
(405 mm) occurred. In the following months little rainfall occurred until month
23 (March 1994). From April to October 1994, rainfall was 268 mm. Finally, there
was no precipitation from November to April 1995, except for December when it
was 36 mm. During May and June 1995, rainfall was 53 mm.

Fig. 2. Rainfall recorded during three years for the guayule study.

Relationship Between Dry Weight, Rubber Content, and Climatic Conditions.
The dry weight, rubber and resin content for the accessions, as a function
of age has been published by Jasso-Cantú et al. (1997). These results
were analyzed further to determine whether a correlation existed among the three
variables and climatic conditions.

The rubber content for accession BG1123 is plotted as a function of dry weight
in Fig. 3. Dry weight increased (or at least remained constant) whereas the
rubber percentage increased or decreased. These changes are season dependent.
The correlation for these variables was positive and highly significant (r =
0.687**). This correlation shows that rubber increases take place mainly during
the autumn-winter periods (September 1993March 1994, and October 1994January
1995). This supports our earlier study (Jasso-Cantú et al. 1997) regarding
the existence of rubber synthesis cycles depending on environmental conditions.
The rubber contents were around 4% for the first 16 months, rising to 8% by
the end of the study period. However, values up to 12% were found during January
1995.

Fig. 3. Rubber content in guayule accession BG1123, as a function of
dry weight during the three years study.

The rubber content and dry weight as a function of cumulative rainfall are
shown in Fig. 4 for accession BG1123. The other accessions showed similar behavior.

The October 1992 to September 1993 period showed a slight decrease in rubber
content from 4.1% to 3.2%; the T-test reported a highly significant difference
(P< 0.01) between these values. During this period the biomass increased
from 4.5 to 45 g per plant. Accordingly, the biomass increased 10 times whereas
the rubber content decreased by 22%. Based on Fig. 1 and 2, we can divide this
growth period into two stages: one covering six months with minimum temperatures,
near and below 0°C and rainfall accounting for 73.3 mm. During this stage
the biomass and rubber remained constant. The second stage had minimum temperatures
between 3°C to 12°C and cumulative precipitation of 440.7 mm; during
this stage biomass increased and rubber decreased.

In the second stage, from September 1993 to March 1994, rubber content increased
from 3.1% to 9.7% and biomass accumulation from 45.1 to 88.8 g/plant. Rubber
increased 300% and biomass 196%. In this stage, the minimum temperature was
between 2°C and 3°C in September and March, and consistently below 0°C
in the remaining months, reaching 5°C in January. Rainfall was 97.7
mm during September and 28.8 mm for the rest of this stage. It should be noted
that there was high rainfall previous to this stage, which promoted biomass
accumulation. On the other hand, the low temperatures combined with the low
rainfall promoted an active rubber biosynthesis raising the average value to
almost 10% in March 1994. The importance of the low temperature and dry conditions
for rubber synthesis (Benedict et al. 1947; Macrae et al. 1986; Appleton and
Van Staden 1989) is evident when the plants were 22 months old.

The third stage covered the March to October 1994 when rubber decreased (9.7%
to 6.9%), and biomass increased (from 88.7 to 135 g). Rubber content reduction
was 28.9% and the biomass increase was 152%. The minimum temperature rose from
3°C (March) to 10°C (June) and then decreased to 5°C in October;
rainfall was 275.3 mm. The relatively high water supply and temperature favored
biomass accumulation, but no rubber synthesis. Thus, the rubber content decreased.
This behavior is similar to that of the first stage.

The fourth stage lasted four months (October 1994January 1995) where
the rubber increased from 6.9% to 11.5% (164% increment), but biomass remained
constant. The minimum temperatures decreased from 2°C to 2°C.
Rainfall was 43.3 mm during the stage. The low temperature and precipitation
again promoted rubber synthesis, with no increase in biomass as discussed in
the second stage.

The last stage covers January to June 1995, where biomass accumulation increased
(135 to 220 g per plant) and rubber content diminished (11.5% to 7%) as in the
first and third stages. Changes were 155% for biomass and 31% for rubber. The
minimum temperatures were below or near 0°C for the first four months and
rose to 8°C by the third stage. Rainfall was 58 mm.

Biomass increased mostly during springsummer, whereas the rubber increased
during autumnwinter season (Fig. 4). Correlations among cumulative rainfall
with rubber content and with dry weight were positive and highly significant
(r = 0.698** and r = 0.928** respectively). The results indicate that rubber
percent reduction in the plant is not due to degradation of rubber or its consumption
when water stress is severe. Rubber reduction was observed during plant growth
(when temperature and rainfall are high).

Fig. 4. Rubber content and dry weight in guayule accession BG 1123,
as a function of cumulative rainfall during a three-year period study.

Correlation Between Relative Variables

The relation of the fractions defined by Eqs. 13 with time, is presented
in Fig. 5 for accession BG1123. The dry weight relative fraction (fDw)
always increased, due to the constant weight gain of the plant discussed in
the previous section. As this variable increased its relative fraction the other
two (rubber and resin) must decrease according to Eq. 4. The rubber fraction
(fRb) share does not present significant
variations with time, suggesting that it may be a constant for each accession.
This variable presents a negative and significant relationship with dry weight
(r = 0.478*), possibly due to the season changes shown in Fig. 4. The
resin fraction fRs always decreases. The
relationship between resin with biomass is negative and highly significant (r
= 0.936*). In young plants, the resin fraction is greater than the rubber
and dry weight fractions. As time elapses, the three fractions reach similar
values at approximately 24 months. When the plants were 32 months old, the dry
weight relative fraction dominated the other two. This indicates that biomass
increase reflects increases in cortex tissue. These results help define the
best time for harvesting, when looking for optimum quantities of the three products
(biomass, rubber, and resin). In this case it was approximately two years. No
defined relationship was found for rubber with resin relative fractions during
the study (r = 0.142).

Fig. 5. Relative variables for dry weight, rubber and resin content
as a function of elapsed time in guayule accession BG1123.

The data obtained with Eqs. 57 for the three accessions (BG1123, BG1132,
and BG11605) were used to construct Fig. 6. Most of the points fall on one line.
The areas around apex corresponded to different conditions of temperature and
moisture. The area close to fRf corresponded
to cold/humid conditions, the area near fRby
represents cold/dry conditions and the area around fSTU
to warm/dry. The area in the middle of the triangle base corresponded to warm/humid
conditions. It is apparent that increasing the temperature (moving from the
left side towards the fSTU
apex) caused a reduction of rubber yield (shifts the points away from the fRby
apex). The combined effect of temperature and moisture is illustrated by dividing
the diagram into two parts, one corresponding to fSTU
from 0 to 0.5, where a displacement on the rainfall fraction (fRf)
axis from 0.5 to 0.1, leads to high rubber yield. The second part of the diagram
corresponded to young plants (less than 14 months) with fSTU
values between 0.5 to 1.0. In this section it is apparent that high values of
thermal units show low rubber yielding values, although no clear trend is shown
with rainfall. This is due to low rainfall during a prolonged period. Rubber
yield is high only for cold/dry conditions, decreasing if temperature or rainfall
increases (Fig. 6). This behavior was confirmed by negative correlations among
rubber yield with the cumulative thermal units and with the cumulative rainfall
for the accessions: BG1123, r = 0.0601** and r = 0.945** respectively;
BG1132, r = 0.455* and r = 0.954**; and BG11605, r = 0.106
and r = 0.948**.

Rubber content, as a response to climatic conditions, is shown in Fig. 7.
This diagram indicated that high rubber content is only found under cold/dry
conditions. This is important because rubber yield production in guayule is
based partly on the rubber content. Correlations among rubber content (fRb%
with cumulative thermal units (fDSTU)
have negative values: BG1123 r = 0.533*; BG1132 r = 0.0527*; and
BG11605 r = 0.574**. Correlations among rubber content (fRb%)
with cumulative rainfall (fDRf)
were also negative with higher r values: BG1123 r = 0.825*;
BG1132 r = 0.802**; and BG11605 r = 0.797**.

CONCLUSIONS

The relationships among the different characteristics show the importance
of temperature and rainfall in rubber synthesis. Temperature was the most important
variable found in defining rubber content. Exposure of shrubs to long periods
with temperatures between 2°C and 4°C promotes rubber synthesis.
The rainfall effect was reflected in the following way: low rainfall levels
improved rubber accumulation, whereas high rain levels did not. However, high
precipitation also improved the biomass production.

The results confirm the existence of rubber synthesis cycles triggered during
the autumnwinter, and rubber content decreased during the spring-summer
season. In contrast, plant dry weight increased mainly during the spring-summer
seasons, particularly when rainfall occurred. These results indicate that biomass
increased both structural and rubber producing cells, which initially do not
contain rubber and later accumulate rubber during the cold/dry season.

According to the results, better agronomic management is possible to optimize
high solid rubber and latex yields for commercial exploitation. Shrub harvesting
may be carried out by the end of the winter when plants are around two-years-old
due to a higher quantities of rubber, latex, and co-products.